The present invention relates to a method for ascertaining a torque and/or a force, which acts on a rotary apparatus, in particular a rotary apparatus of a coordinate measuring machine (CMM), and which is directed across an axis of rotation of a rotor of the rotary apparatus, and to a method for ascertaining a phase angle of the center of gravity of a mass on a rotor of a rotary apparatus.
Rotary shafts, in particular highly precise, air-borne rotary tables, are often sensitive to loads with a tilting moment. Overloading an air bearing may lead to the air gap collapsing and the two opposing bearing surfaces contacting one another. In this case, the rotary shaft must not be moved any further because otherwise damage may arise in the air bearing or the bearing jams.
Rotary shafts mounted on a rolling bearing must not be overloaded either. Although rolling bearings tend to jam less, the rolling bearings are, as a rule, manufactured highly precisely for accurate rotary shafts. In the case of rolling bearings, overloads lead to plastic deformations/wear and tear, e.g. of the rolling elements, as a result of which the accuracy is subsequently no longer provided. A rotary shaft mounted on a rolling bearing may be damaged, in particular if the overload is present for a relatively long time.
Spindle examinations of rotary tables mounted on a rolling bearing show that these often have an outstanding accuracy in the delivered state, but exhibit significant movement errors after a few years of operation. Here, the deterioration of the rotary shaft often goes unnoticed, as a result of which unreliable measurement results arise.
By way of example, the problem also affects rotary swivel joints, in which tilting moments or torques arise perpendicular to the axis of rotation, for example as a result of the installation position thereof or the employed sensors and probing systems. Often, the bearings of rotary swivel joints in metrology are designed to be relatively weak in this case, for reasons of weight or space.
If a rotary apparatus at the customer needs to be replaced, for example because the coordinate measuring machine (CMM) no longer passes the acceptance inspection, the reason for the wear and tear often remains unclear, or it is usually not possible to prove that the customer has overloaded the rotary shaft.
Some rotary tables, such as e.g. the RT-AB by Zeiss, have complicated and expensive bearing monitoring which, initially, is only employable for air-borne rotary shafts.
As a rule, tilting moments in the case of rotary shafts also cause faults in the rotational angle measuring system, in addition to deviations from the ideal rotational movement (movement error). In order to make at least the angle measuring system largely independent of the influences of the tilting moment, use is often made of expensive angle measuring systems with a dedicated bearing. Then, such angle measuring systems with a dedicated bearing are often afflicted by friction, as a result of which hystereses may arise.
It is an object of the invention to solve one or more of the aforementioned problems. In particular, a method for ascertaining a torque acting on a rotary apparatus or a force acting on a rotary apparatus should be specified. With the aid of the method, it should be possible to determine overloading of the rotary apparatus, in particular of a bearing of the rotary apparatus.